In air-fuel ratio (hereinafter referred to as AFR) control systems using an AFR sensor for continuously detecting AFR from the oxygen concentration of the exhaust gas, an air-fuel mixture can be obtained having any AFR over a wide range. This AFR includes, but is not limited to, the theoretical AFR.
For this purpose, a basic fuel injection amount is determined according to the engine load and speed, and the fuel injected from a fuel injector is feedback corrected based on the difference between a real AFR detected by the AFR sensor and a target AFR.
In this type of AFR control system the precision of the AFR sensor is of vital importance, and if it deteriorates, the precision of AFR control is also poorer.
A method of detecting AFR sensor deterioration is disclosed in, for example, Tokkai Sho 62-186029 published by the Japanese Patent Office.
According to this detection method, it is judged whether or not a feedback correction coefficient computed from the difference between the output of the AFR sensor and a target AFR is within a predetermined range. If the value of this coefficient lies outside this range, it is judged that the AFR sensor has deteriorated.
However, the feedback correction coefficient may lie outside the predetermined range due not only to deterioration of the AFR sensor, but also to other factors such unevenness or deterioration in the performance of the fuel injector or the air flow sensor which detects the intake air volume of the engine. Using this method, therefore, there was a possibility that the AFR sensor was judged to have deteriorated even when it had not deteriorated.